1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device and a manufacturing method thereof, and more particularly to a flip chip type nitride semiconductor light emitting device and its manufacturing method wherein a p-metal layer of the nitride semiconductor light emitting device, which has a flip chip structure, comprises three layers, namely, a bonding force providing layer, a reflective electrode layer, and a cap layer, so as to enhance a bonding force between the p-metal layer and a p-type nitride semiconductor layer of the device, to improve efficiencies of reflection and electric current diffusion, and to reduce contact resistance, resulting in enhanced brightness and driving voltage qualities of the device.
2. Description of the Related Art
Currently, in order to enable such a nitride semiconductor light emitting device to be utilized as a next-generation lighting apparatus, there exists a requirement of developing a nitride semiconductor light emitting device so as to have a high-brightness quality. As a prior art technology for satisfying this requirement there has been proposed as follows.
FIG. 1 is a sectional view illustrating one example of a conventional nitride semiconductor light emitting device. As shown in FIG. 1, the exemplary conventional nitride semiconductor light emitting device comprises an n-type nitride semiconductor layer 12, an active layer 13 with a multiple quantum well structure, and a p-type nitride semiconductor layer 14, which are successively stacked on a substrate 11 in multiple layers. The p-type nitride semiconductor layer 14 and the active layer 13 are partially removed at their side regions, respectively, so that a part of an upper surface of the n-type nitride semiconductor layer 12 is exposed to the outside. On the exposed upper surface region of the n-type nitride semiconductor layer 12 is mounted an n-side electrode 18. Afterwards on the p-type nitride semiconductor layer 14 is formed a p-metal layer 15, which is for forming an ohmic contact and for improving injection efficiency of electric current, a p-side bonding electrode 19 is formed on the p-metal layer 15. The p-metal layer 15 has a light transmission property and acts to reduce contact resistance, thereby serving to improve injection efficiency of electric current. In general, as such a p-metal layer is employed a transparent conductor layer consisting of Ni/Au double layers.
The Ni/Au double layers are metal layers having a relatively good light transmissivity. As they are deposited in a very thin layer of 100 Å or less, light produced inside the light emitting device is emitted to the outside through metals. Although it is preferable that the Ni/Au double layers are deposited in a thickness as thick as possible in order to improve injection efficiency of electric current, due to the fact that a transparent electrode is formed by using metals, such a thick thickness may be an obstacle in view of desired light transmittance quality. Even if the light transmittance quality can be enhanced through additional heat treatment process of the light emitting device, since a resulting light transmittance ratio is approximately 60% at the most, such an increase in thickness of the Ni/Au double layers may cause deterioration in brightness of the nitride semiconductor light emitting device. Therefore, the thickness of the Ni/Au double layers are inevitably restricted, and thus have a limitation in improvement in injection efficiency of electric current.
In order to solve a problem of the conventional nitride semiconductor light emitting device shown in FIG. 1, there has been proposed in the prior art a flip chip type nitride semiconductor light emitting device as shown in FIG. 2. This flip chip type nitride semiconductor light emitting device, in the same manner as the above described conventional nitride semiconductor light emitting device shown in FIG. 1, comprises an n-type nitride semiconductor layer 22, an active layer 23 with a multiple quantum well structure, and a p-type nitride semiconductor layer 24, which are successively stacked on a substrate 21 in multiple layers, and the p-type nitride semiconductor layer 24 and the active layer 23 are partially removed at their side regions, respectively, so that a part of an upper surface of the n-type nitride semiconductor layer 22 is exposed to the outside. Then, on the exposed upper surface region of the n-type nitride semiconductor layer 22 is mounted an n-side electrode 28, and on the p-type nitride semiconductor layer 24 is mounted a p-metal layer 25. The p-metal layer 25 is made of Al, Ag, and the like having a good reflectivity. After that, a p-side bonding electrode 29 is mounted on the p-metal layer 25. In use, by reversing the obtained nitride semiconductor light emitting device, the n-side electrode 28 and the p-side bonding electrode 29 are connected to conductive patterns provided on a submount 201 by interposing bumps 202, respectively. In case of this flip chip type nitride semiconductor light emitting device, the p-metal layer 25 is made of the above mentioned metals having a good reflectivity in order to effectively reflect light, which is intended to proceed into directions of the electrodes, thereby causing the light to be emitted to the outside through the transparent substrate 21. In this way, it is impossible to improve brightness of the light emitting device and to facilitate dissipation of heat, which is generated inside the light emitting device by the electrodes, to the outside.
The p-metal layer 25, however, has a problem in that its constituent Ag presents a very low bonding force relative to the p-type nitride semiconductor layer 24, thereby causing generation of poor products and making it difficult to ensure a stable contact resistance quality. Although other metals including Al, Ni, Ti, Pt, and the like is usable to constitute the p-metal layer 25, these metals have a low reflectivity compared to Ag.
Therefore, there still exists in the art a requirement of developing a new p-metal layer, which has a good bonding force relative to a p-type nitride semiconductor layer, presents a high brightness quality, and can achieve stable electrical qualities, such as electric current diffusion and contact resistance.